Homogeneous catalysis selectivity

According to these, for purely aliphatic ketones the highest enantioselectivities are achieved for methyl ketones with a second, branched-chained alkyl substituent (84-94% ee). The value of 75 % ee obtained with the straight-chain hexan-2-one is, to the best of our knowledge, in any case better than anything achieved to date with nonenzymatic systems and homogeneous catalysis. Higher selectivities have been reported for reductions with stoichiometric amounts of chiral borohydrides (e.g. 80 % ee for the reduction of octan-2-one) [20]. 

Polymer-supported chiral catalysts have likewise been prepared in order to obtain access to reusable systems (cf. Section 3.1.1, especially Section 3.1.1.3). For example, copolymerized functionalized BINAP [160, 161] could be applied in the enantioselective hydrogenation of olefinic substrates (up to 94 % ee). Similarly, the copolymerization of vinyl-BINAPHOS with styrene derivatives led to a heterogenized auxiliary which made it possible to hydroformylate styrene and vinyl acetate (Rh catalysis) with selectivities and enantioselectivities close to those provided by the parent homogeneous catalytic system [162]. 

Homogeneous catalysis of selective functionalization of alkane and alkenes by dioxygen... 

Give a brief discussion of the use of homogeneous catalysis in selected industrial manufacturing processes. 

In heterogeneous catalytic reactions such as cracking and reformation, this is routinely done on a very large scale. In enzymatic catalysis too, selective oxidation of hydrocarbons is well established. In homogeneous catalytic systems, such reactions are rare, but they continue to be of great interest because alkanes such as methane are abundantly available as potential feedstocks. 

The repeated use of the biocatalyst is often necessary to reduce the catalyst costs. Soluble enzymes can be easily recovered from the reaction mixture using an ultrafiltration step [18,96,97], thereby combining homogeneous catalysis wifli selective enzyme recovery and reuse. Ultrafiltration can be performed from a microliter to a multi cubic meter scale with commercially available ultrafiltration membranes and equipment. For first experi-... 

A review of industrial processes that employ homogeneous catalysts is by Jennings, ed. (Selected Developments in Catalysis, Blackwell Scientific, 1985). Some of those processes are ... 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

The potential for the use of catalysis in support of sustainability is enormous [102, 103]. New heterogeneous and homogeneous catalysts for improved reaction selectivity, and catalyst activity and stabihty, are needed, for example, new catalytic materials with new carbon modifications for nanotubes, new polymers. 

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